1. Field of the Invention
This invention relates to a method for preventing the reflection of the surface at a metal or semi-metal of a light absorbing body.
2. Description of the Prior Art
Recently, there has been significant progress in photoetching techniques for making electronic parts of integrated circuits or conversion elements for converting optical energy into other energy or in the technology of working a thin film by a visible or infrared laser device. Consequently, it has been required to control the reflecting light at the surface of a body which absorbs a light (hereinafter called a light absorbing body).
In general, the surface reflecting power of a metal or semi-metal having a light absorbing property in the visible light region is as follows: Silver 94-98%, Al 88-91%, Au 35-98%, Cr about 50%, Bi 40-60%, Ge 40-60%, Si about 40%, and Cu 50-97% which show significantly high reflecting power.
In making the parts of an optical device or energy conversion elements using metal or semi-metal as a material thereof, since the light reflecting powers of these materials are high, it is difficult to effectively utilize the energy of the incident light because light reflection badly effects the properties of the device or elements.
It has been proposed to coat the surface of a light absorbing body with a dielectric substance which does not absorb the light so as to prevent light reflection. One of such methods is shown in "Vacuum Deposition of Thin Films" by L. Holland published by John Wiley & Sons, 1958, in which is reported coating a film of MgF.sub.2, which is a dielectric, with a thickness of one fourth of the wavelength on the surface of a light absorbing film of the mixture of Fe.sub.2 O.sub.3 and CaSiO.sub.3. The other method is shown for example in "Non-metal absorbing film and reflection-prevention" by Katsube, published in Applied Physics 1968 37, Pages 225-230, in which on the light absorbing film of Cr.sub.2 O.sub.3, two dielectric layers of CeO.sub.2 and MgF.sub.2, respectively, are coated for preventing the reflection of light. Further, a method is shown in U.S. Pat. No. 3,560,994 in which the surface of a metal is coated by a dielectric film so as to decrease the reflection of light at the surface of the metal. In this patent, the surface of Bi(bismuth) having complex refractive index n-ik=2.07-3.93i is covered by a dielectric film having refractive index n.congruent.4, so as to decrease the reflecting power of the surface of the Bi film. As for the material having the refractive index of about n.congruent.4, Si(n= 4.5) and Ge(n=4.4) have been indicated as suitable. Further, when visible red is assumed as a reflection-preventing region, it was suggested that Ge is a suitable material for covering the bismuth surface. According to a kind of vapor-deposition method for vapor depositing Ge on the base plate of bismuth, it is possible to make Ge as an absorbing material in the visible red region. However, in this U.S. Pat. No. 3,560,994 only dielectric substances have been considered as the material for deposition on the base plate, and consequently no consideration has been given to imaginary number portion k of the refractive index of a material to be vapor deposited. Moreover, in a reactive vapor deposition method in which the deposition is made in a vessel in which an active gas such as oxygen or sulphur has been introduced, vapor deposited Ge has a property of a dielectric material in the visible red wave length region. Therefore, from this patent it should be considered that the surface of a metal is covered only with a dielectric material for preventing light reflection at the metal surface.
However, in the above mentioned reflection-preventing method, the surface of a light absorbing body is covered by a dielectric film, in which the imaginary number K=0 in the complex refractive index n-ik, so that the conductivity and thermal conductivity of the surface of the light absorbing body are deteriorated and there is difficulty in subjecting it for the photo-etching process.
For overcoming said defect, it is preferable to cover the surface of a light absorbing body with the film of a second light absorbing body having the same property as that of the base light absorbing body for preventing reflection. However in using such light absorbing body as the reflection-preventing film the following two conditions must be considered in comparison to the case in which a dielectric body is used for the purpose of preventing the reflection.
One condition is that the imaginary number k in the complex refractive index n-ik of the reflection-preventing film can not be neglected as the film is of a light absorbing body. In the complex refractive index, real number n represents the deviation of the phase of the incident light and the imaginary number k represents the attenuation of the amplitude of the incident light and it is called an attenuation number. Therefore, when a material having a large attenuation number is used for the reflection-preventing substance, the amplitude of the light reflected at the interface between the base plate and the reflection-preventing film and passing out from the film shall be significantly attenuated.
The second condition is that since both the reflection-preventing film and the base plate are of light absorbing bodies, respectively, the real numbers and the imaginary numbers of the complex refractive indices of both bodies, respectively, contribute to the fresnel reflection at the interface of both bodies.
Recently, the use of a light absorbing body for preventing reflection has been reported. One report is shown in "The condition for reflection-preventing method of precious metal thin film" by Mizuhashi et. al., 1971, Preminary presention of the 18th Applied Physics of Allied Science Lecture Meeting. This method is to provide a film of a suitable thickness having a slight absorbing property on the surface of a silver film. However, the complex refractive index of the metal used therein shows an attenuation number less than 0.1 and when calculating the condition of the reflection-prevention, the attenuation number is assumed to be zero, so that the metal is treated as a dielectric body. In fact, when the attenuation number is less than 0.1, the substance has the property of a dielectric rather than light absorbing body, so that there is no advantage of using a light absorbing body. Furthermore, in the existing metals there is no metal which has the imaginary number k of the complex refractive power less than 0.1.
In "Prevention of reflection by a metal film" by Sawaki, Investigation of reflection-preventing film, Chapter 6, October, 1960, Osaka Industrial Laboratory, a method of using a metal film as the reflection-preventing film is described. In this article, the reflection-preventing film of a metal is provided on the base plate of glass which is a dielectric body, so as to prevent reflection at the surface of the glass base plate. In case the external medium of the light incident side is air, i.e. n=1, the metallic reflection-preventing film for descreasing the reflecting power on the base plate of the light coming from the medium should have values of significantly less than 1 for optical constants n and k, respectively. In reality, there is no metal having such optical constants. Moreover, when the incident light comes from the glass plate side, the thickness of a metal film for preventing the reflection of light on the surface of the glass base is 1/1000-1/100 of the wavelength of the incident light, the metal film becomes too thin if the wavelength to be used is within or close to the visible region and therefore it is not possible to obtain an uniform thin reflection-preventing film.
In these two articles, a light absorbing body is considered to be a reflection-preventing film, but in the former article, the attenuation number is neglected and in the latter article the solution of the reflection-preventing film is a metal which does not exist. And also there has been no consideration of using a thin film of a light abosrbing body to prevent the reflection at the surface of the base plate of a light absorbing body. This was caused by the fact that the conditional equation for the light-prevention becomes complex if the base plate is of a light absorbing body.